The present invention relates to data networking and more particularly to systems and methods for flow control.
The Fibre Channel standard defines a bi-directional link protocol commonly used to connect computers to disk drives and other peripherals. A typical Fibre Channel link may have a bandwidth of 1063 Mbps and a span of up to 10 kilometers.
One typical application of Fibre Channel is interconnecting computer CPUs with arrays of disk drives in large scale computing centers, as would be used in, e.g., financial transaction processing. For reasons of fault tolerance, it is desirable to locate redundant storage resources at remote locations. The advent of high data rate metropolitan optical networks makes it possible to implement so-called storage area networks (SANs) that span over a much longer distance than 10 kilometers.
It would be preferable to apply the widely prevalent Fibre Channel standard to communication across SANs and therefore minimize the need to redesign computing center equipment. A problem arises, however, in that most Fibre Channel devices available now assume link distances no more than 10 kilometers while it is desirable to locate SAN nodes much further apart, e.g., hundreds of kilometers.
The Fibre Channel standard defines a flow control scheme that maximizes data throughput while preventing the transmitter from sending more data than the receiver is currently able to process. For the most prevalent classes of Fibre Channel devices, the standard utilizes a buffer-to-buffer credit management scheme. When a link is set up, the two ends exchange information about the size of their receiver buffers. A Fibre Channel receiver port sends a ready signal indication after each received frame but only if there is sufficient buffer space to accommodate the largest possible frame of new data. The transmit port counterpart uses the ready signal indication and its knowledge of the receiver port's buffer size to determine whether or not to transmit a frame. This scheme works well over relatively short distances but breaks down over larger distances because of the long delay between sending a frame and receiving a ready indication in response.
What is needed are systems and methods for managing flow control in Fibre Channel links that may extend over large distances.